An endoscope is widely used in an industrial field, a medical field, and the like. The endoscope consists of an operation section and an insertion section. The insertion section is flexible and inserted into an object. The insertion section contains an optical fiber bundle (light guide) for guiding illumination light rays from a light source device. A distal end surface of the insertion section has an illumination window, an observation window, and the like. An illuminating optical system is disposed between the illumination window and the optical fiber. An imaging optical system is disposed behind the observation window. In a case of a fiber scope, an image formed by the imaging optical system is taken out by the optical fiber bundle (image guide). In a case of an electronic endoscope, an image sensor is disposed behind the imaging optical system.
Depending on the intended use, the endoscope is used to observe objects having various kinds of shapes. The objects to be observed by the endoscope may be planar, spherical, or tubular. For example, objects to be observed by a medical endoscope have an approximately spherical shape such as an interior of stomach, and an approximately tubular shape such as an interior of esophagus or bronchi.
In recent years, a wide angle lens has been adopted for the imaging optical system of the endoscope. In accordance with this, it has been demanded that the illuminating optical system also has a wider angle. Namely, it has been demanded that the illuminating optical system increases its illumination area. Additionally, the illuminating optical system has been required to illuminate an object to be observed with uniform illuminance distribution.
In response to the demand described above, an illuminating optical system having at least one positive lens is disclosed in Japanese Patent No. 3020074. The illuminating optical system almost satisfies a condition expressed by H=F·θ, in which a focal length is denoted by F, a height of light ray incident on the optical system in parallel with an optical axis (hereinafter referred to as incident light ray height) is denoted by H, and an angle at which the light ray having the incident ray height H exits from the optical system is denoted by θ. Illumination light rays emitted from the optical fiber bundle are converged once, and then diverged inside the positive lens, and therefore it is possible to illuminate a wide area. Further, when a spherical body is illuminated, the illuminance on the spherical subject is increased from the center toward the marginal portion of the spherical body. Additionally, the light rays which are to travel toward the marginal portion of the spherical body through the lens are irregularly reflected and disappear or totally reflected by the inside of an outer circumferential surface of the lens. Accordingly, it is possible to illuminate the spherical body with approximately uniform illuminance distribution, and further illuminate the planar body and the tubular body with favorable illuminance distribution.
Additionally, an illuminating optical system disclosed in Japanese Patent Laid-Open Publication No. 2002-244050 is provided with at least one light distribution lens. One surface of the light distribution lens is partitioned into plural concentric ring zones. A convex surface and a concave surface are continuous at a boundary of these ring zones. One of the convex surface and the concave surface has a shape obtained by shifting part of a reference curved surface in the optical axis direction. The other of them has a shape symmetrical to the reference curved surface with respect to a surface vertical to the optical axis. The light distribution lens has a shape similar to that of a Fresnel lens, and therefore it is possible to decrease the thickness of the lens.
However, in the illumination optical system disclosed in Japanese Patent No. 3020074, a tilt angle of an incident surface in a marginal portion of the lens is made to be wide, such that all light rays incident on the lens pass through the lens and then are focused on the optical axis. The lens is produced by molding. The molding is performed by pressing a spherical preform made of glass. In the molding, since the tilt angle in the marginal portion of the lens is wide, it is difficult to keep precision, and further the molding takes much time.
In the illumination optical system disclosed in Japanese Patent Laid-Open Publication No. 2002-244050, the lens has a complicated shape having an acute concentric part. Accordingly, it is difficult to keep precision in the molding, and the molding takes much time.
Additionally, in both illumination optical systems disclosed in the above patent documents, the light rays which pass through the illumination optical system are once converged and then diverged, and therefore a converging point at which light energy is high is formed. Since the converging point is outside the illumination optical system, if the illumination window approaches an interior of stomach or the like, the interior of stomach or the like may be damaged in some cases. Further, unless the illumination window is away from the object to be observed to some extent, it is impossible to illuminate the object with a wide light distribution angle.